The present invention relates to an inverter apparatus for controlling the speed of an induction motor variably.
As a method of controlling an inverter for driving the induction motor so that the induction motor is operated at variable speed, there is known a V/f fixed control method of controlling an output voltage (V1) of the inverter in proportion to a primary frequency (f1) of the inverter. This method has a problem that when a load is applied, an induced voltage (Em) of the induction motor is reduced because of a voltage drop across a primary resistance (r1) of the induction motor, so that a magnetic flux of the induction motor is made small and accordingly a maximum torque is reduced.
In order to increase a torque in a low and medium speed area, a general inverter includes torque boost function. When a large start torque is required, a boost voltage is set up to a high voltage in a low speed area and the boost voltage is added to a V/f fixed voltage command (induced voltage command Em*) to produce an output voltage command of the inverter. However, when the boost voltage is increased, over-excitation occurs in no load. When the over-excitation occurs, the magnetic flux of the induction motor is saturated and accordingly an excitation reactance is reduced to thereby increase an excitation current. Consequently, the temperature of the induction motor rises or the current of the inverter is increased excessively, so that there is the possibility that over-current protection function or over-load protection function is operated to be tripped.
A method of suppressing the over-excitation is described in, for example, JP-A-7-163188. In this method, a command for setting up a frequency to zero is issued before start of operation and a DC current is supplied to the induction motor. An output voltage of the inverter at the time that a current of U-phase becomes equal to an equivalent of a design value of the excitation current is set up as a torque boost voltage xcex94Vz0 at the time that the frequency is 0 Hz.
In the above method, since a torque boost voltage is set up so that the current in no load is equal to a rated excitation current (design value of excitation current), no over-excitation occurs. In this case, however, the voltage drop across the primary resistance is increased when the induction motor is loaded. As a result, there is a problem that the induced voltage (magnetic flux of motor) is reduced to thereby decrease an output torque. In this manner, when the torque boost voltage is made high, the torque is increased, while over-excitation occurs when the load is light. Conversely, when the torque boost voltage is made low, the over-excitation does not occur, while there is an antithetic problem that the torque is not increased.
It is an object of the present invention to provide an inverter apparatus suitable for prevention of over-excitation even when a torque boost voltage is set up to be high in order to obtain a large start torque in a general inverter.
In order to achieve the above object, the inverter apparatus according to an aspect of the present invention comprises detection means for detecting an excitation current of the induction motor, setting means for setting a limitation level of the excitation current, torque boost voltage command means for producing a torque boost voltage command in response to a frequency command of the inverter apparatus, and torque boost voltage compensation means for changing the torque boost voltage command so that the detected excitation current value is smaller than or equal to the excitation current limitation level.
The torque boost voltage compensation means includes limiter processing means and inverts the torque boost voltage command. The inverted torque boost voltage command is limiter-processed as a lower limiter value of the limiter processing means to produce a compensation value of the torque boost voltage command.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.